Wobbled radio carrier communication system



June 1947- c. B. H. FELDMAN $2,422,554

WOBBLED RADIO CARRIER COMMUNICATION SYSTEM Filed July 12, 1944 3Sheets-Sheet 1 TRANSMITTED SIGNAL E b 0 k "I & I

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WOBBLED RADIO CARRIER COMMUNICATION SYSTEM Filed July 12, 1944 5Sheets-Sheet 5 AUDIO AMP Al C

DETECTOR FIG. 7

RECEIVE lNl ENTOR C 8. H FELDMAN Patented June 24, 1947 WOBBLED RADIOCARRIER COM- MUNICATION SYSTEM Carl B. H. Feldman, Summit, N. 3.,assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y.,a corporation of New York Application July 12, 1944, Serial No. 544,493

11 Claims. 1

This invention relates to radio signaling and, more particularly, tomethods and systems for obtaining a high degree of accuracy in receivingand recording radio signals.

Ordinary narrow band telegraph transmission over radio circuits issubject to two main troublesome factors which tend to produce errors inthe recording of the received signals. One of these factors is fadingwhich causes the signals to vary in level and the other factor iscrashing static which produces extreme variations in the noise level.When the transmitted signals are received by a radio telegraph receivingsystem of the type in which the received signals efiect the operation ofa two-position electromechanical device, continual adjustments must bemade in order to prevent the above-mentioned factors from causing errorsin the recording of the signals. These adjustments can be made in eitherthe sensitivity of the electromechanical device or in the level of thesignals applied to it so that its operate and release values will liecontinuously in the interval between the signal level and the noiselevel when such an interval exists. Vfith this type of receiving system,completely errorless operation is impossible when fading carries thesignal level below the noise level or when noise crashes exceed thesignal level.

Errors due to fading can be rendered less frequent by employing someform of diversity reception. Since fading which carries the signal levelbelow the noise level is due usually to interference produced by thesignaling radiant energy traveling through the ionosphere over amultiplicity of space paths, any means for separating the duplicatesignal waves received over these various space paths and for preventingdestructive interference between them will reduce this type of fading.One form of such a separation of the multiple signal waves is aseparation based on a time standpoint. This is made possible by the factthat the multiple space paths generally possess slightly differenttravel times which produce various delays having diiferences of amillisecond or more, in the arrival of the duplicate signal waves.

Heretofore, one way of accomplishing such a time separation of thereceived duplicate signal waves has been to transmit a short pulse ofabout 0.3 millisecond duration in place of the 3S milliecond markingsignal commonly employed in printing telegraphy. This results in thereception by the radio receiving system of a succession of short pulsesbetween which there can be no interference. Transmission of these shortpulses requires a Wider transmission hand than is required fortransmission of the longer pulses and is consequently subject to ahigher degree of noise. However, if the peak pulse power is increasedsufficiently to make the average power the same as that employed intransmitting the 30 millisecond type of pulses, no penalty will resultfrom the attendant higher noise. Y

For example, if the signaling speed of the transmitter is maintained thesame as that used duringtransmission of the 30 millisecond type ofpulses, then, when the duration of each pulse is shortened to 0.3millisecond, the peak power can be increased one hundred fold Withoutincreasing the average power used by the transmitter. Since thisrequires that the band width of the receiver be increased one hundredfold, the noise power will be one hundred-fold greater. As was statedabove, no penalty will result from .the higher noise because the pulsesignal-to-noise ratio, which is determined bythe values of the noise andthe peak pulse power, will remain the same as it Was in the case of thetransmission and reception of the 30 millisecond pulses.

Accordingly, it is an object of this invention to obtain a high degreeof accuracy in receiving and recording radio signals by preventingdestructive interference between signal waves traveling over differentspace paths.

'It is also an object of this invention to improve the transmissioncharacteristics of radio signals.

An additional object of this invention is to provide a radiocommunication system with improved means for transmitting and receivingradio signals.

Another object is to provide a radio communication system with improvedmeans for separat ing from a time standpoint duplicate signal wavesreceived over a multiplicity of space paths. I

A further object is to provide a radio communication system withimproved means for avoiding the harmful effects which noise currentstend to produce in the recording of received signals.

Still another object is to provide a radio communication system withimproved means for converting duplicate signal waves having a timediversity separation into waves having a frequency diversity separation.I

These and other objects of the invention are attained by modulating thefrequency of a continuous wave of radiant energ over a wide-bandintermittently in a saw-tooth manner in accordance with the signals tobe transmitted. For example, if the signals to betransmitted are marl;-ing and spacing telegraph signals, then, when TIM the continuous wave ismodulated intermittently in a saw-tooth manner, each tooth wouldrepresent one type of signal, such as a marking signal, and eachinterval between the teeth would represent a different type of signal,such as a spacing signal. The resulting modulated continuous wave isthen radiated through space. Sinc the rad;- ated energy travels overmultiple space paths having different travel times, the receiver isconsequently provided with duplicate, o multiple, signal waves having atime diversity characteristic. A species of frequency diversity mightalso be considered to be present because not all of the wide band ofmodulated frequencies is suppressed simultaneously by interferencefading.

As each space path is affected by different degrees of fading anddifferent types and amounts of interference, some of the receivedmultiple signal waves are less suitable than others for use by thereceiver in actuating the recording device. In order to effect aselection of the more desirable duplicate signal waves and to insurethat the recording device will be operated by signal waves of at least aminimum strength, all the received multiple signal waves are detected atthe receiver by detecting means employing a beating oscillator havin itsfrequency modulated over a wide band in a saw-tooth manner similar tothe saw-tooth frequency wave generated at thetransmitter with which itis synchronized. The audio frequency output of the detector is applied,after amplification, to a plurality of narrow band-pass filtersconnected in parallel. The limits of each filter are different from thelimits of all the othe filters and the pass-bands of all the filters arecontinuously distributed in an overlapping manner over the frequencyrange of the signaling energy applied thereto. For example, in apreferred embodiment of the invention in which the passbands of thesignal filters are continuously distributed over a range of 1140 cycles,forty overlapping filters are used, each having a pass-band of 57cycles.

The currents passed by the filters are separately rectified by aplurality of rectifiers which have their outputs separately connected toa scanning device which determines whether any of the rectified filteroutputs have a magnitude above a preassigned minimum value desirable forproper operation of the recorder. If at least one of the rectifiedfilter outputs has a magnitude in excess of the preassigned minimumvalue, then the recording mechanism will be actuated to effect therecording of a marking signal. Thus, in accordance with the invention,the diversity channels of the above-described frequency modulationdiversity radio communication system are scanned or sampled during thereception of each signal to insure that, when the recording device isoperated by a marking signal, said marking signal will have at least aminimum strength. Continuous wave interference is suppressed because thepass-bands of the filters are not sufficientl wide to allow the varyingfrequency currents, produced by the continuous wave interference and thevariable frequency oscillator, to build up to full amplitude.

These and other features of the invention are more fully described inconnection with the following detailed description of the drawings inwhich:

Fig. 1 is a diagram of the characteristics of radio-telegraph signalstransmitted in accordance with this invention;

Fig. 2 illustrates the time diversity separation obtained when thesignals shown in Fig. 1 are received over two space paths havingdifferent travel times;

Figs. 3 and 4 represent detected signal waves having a frequencyseparation instead of the time separation illustrated in Fig. 2;

Fig. 5 indicates the duration and occurrence of scanning operationsperformed upon the rectified output of one of the narrow band-passfilters;

Fig. 6 shows a radio transmitting station for transmittin radiotelegraph signals having the characteristics illustrated in Fi 1;

Fig. 7 illustrates a radio receiving station for receiving and recordingsignals transmitted by the radio transmitting station shown in Fig. 6;and

Fig. 8 is a block schematic diagram of a complete radio telegraph systememploying the radio transmitter of Fig. 6 and the radio receiver of Fig.7.

In Fig. 6, a tape perforator l forms permutation code signals,represented by the holes 2, in transmitting tape 3. The transmittingtape 3 is fed in a well-known manner into a transmitting teletypewriterG. The tape 3 is fed in the direction indicated by the arrow and, justbefore it enters the teletypewriter 3, it passes under an automatic stoplever 5 which normally rests on the upper arm of a control contact 6 asis described in Patent 2,055,567 granted September 29, 1936, to E. F.Watson. The disclosure of this Watson patent is incorporated herein byreference as a part of this specification. Control contact 6 isconnected through a manually operable switch l and through the windingof a magnet 8 to a battery 9. The armature Iii of magnet 3 forms a stoparm for stopping rotation of a stop cam ll mounted on a main drive shaft52. This main drive shaft I2 is driven by meam of a gear Wheel 53,fixedly mounted on the shaft i2, in mesh with a worm M carried on ashaft 5 driven through a friction clutch It by a motor ll. Motor H issupplied with current from a source 45) and its speed can be regulatedby manually adjusting the rheostat 4!. A. manual switch 4.2 is providedfor stopping and starting motor H.

A rotary distributor brush arm H3 is fixedly mounted on the main driveshaft I2 for rotation therewith. The brush arm I8 is equipped with twobrushes adapted to sweep over a solid inner commutator ring l9 and asegmented outer ring 28. The outer ring 29 is provided with fivesegments corresponding to the five units of a permutation code, a stopsegment, and a start segment, each segment being insulated from theother segments. The two rings !9 and 20 constitute the face-plate of atransmitting distributor 2| comprising, in addition to this face-plate,the rotary brush arm l8. Each of the five code segments of ring 29 areseparately connected to one of the five marking contacts 22 which areadapted to b selectively operated by the five contact tongues 23. Thesefive contact tongues 23 are selectively operated in the manner describedin the above-mentioned Watson patent in accordance with the permutationcode signals 2 punched in the transmitting tape 3.

When any of the contact tongues 23 are operated to their associatedmarking contacts 22, a path is closed from battery 24, over the operatedtongues 25 and their associated contacts 22, and then to thecorresponding segments of ring 2! The brush arm is, in rotating,electrically connects seriatim the above-mentioned segments of ring 2!}to the solid inner ring 19 which is connected by conductor 25 to thewinding of a relay 26. Thus, the battery 24 is connected through theoperated contact tongues 23 and over the above-described path to thewinding of the relay 26 to hold its armature against its left-handcontact. As is shown in Fig. 6, the armature of relay 26 is normally inengagement with its left-hand contact thereby shunting a resistance 21for a purpose explained hereinafter. In the case of a spacing signal, nocurrent will be supplied to the winding of relay 26 which willaccordingly release its armature which will move to its right-handcontact thereby opening the shunt across the resistance 21. A battery 34is connected to both the stop and the start segments of ring 20 so thatthe shunt across the resistance 21 will always be closed while the brusharm I8 is sweeping over the stop and the start segments.

The radio portion of this transmitting station includes a variableelectronic oscillator 28 of an appropriate design. A variable condenser29 is suitably connected to the oscillator 28 for cyclically varyin thefrequency of the current generated by the oscillator 28. The capacitanceof the capacitor 29 is varied by a sliding member 30 which has one endafiixed to the movable plates of the condenser 29 and which has a camfollower 3! mounted on its other end. The cam follower 3| is adapted toride along the edge of a rotating cam 32 and is held in contacttherewith by a coiled spring 33.

Cam 32 is fixedly mounted on the main drive shaft I 2 for rotationtherewith. Six risers are formed in the edge of cam 32 of which fivecorrespond to the five permutation code segments of ring 20 and theremaining riser corresponds to the "stop segment of ring 29. These sixrisers each have a periphery designed to vary the capacitance ofcapacitor 29 in such a way as to cause the frequency of the currentgenerated by the oscillator 28 to vary over a wide range in a saw-toothmanner. Instead of a start riser, the cam 32 is provided with a portionhaving an edge evenly spaced from the center of cam 32 to hold the camfollower 3i steady while the brush arm I8 is sweeping over the startsegment. During this period, oscillator 28 will generate a wave ofsteady frequency which is the "start signal. In the position of rest,the cam follower 3| is shown to be practically at the top of the "stopriser so that the oscillator 28 will be generating waves havingapproximately the maximum frequency.

The current thus generated by oscillator 28 is picked up by a coil 35and is supplied by means of the transformer 36 to the control grid of anelectronic power amplifier 3! to vary the frequency of its oscillationsover a wide range in a saw-tooth manner. The resulting frequencymodulated wave is supplied through a, transformer 38 to the supplycircuit for the transmitting antenna 39 which radiates the frequencymodulated wave into space.

In order to key the power amplifier 31 for effecting an intermittentmodulation of the continuous wave in accordance with marking and spacingsignals, the resistance 21 is connected into the cathode return circuitof tube 31. When the system is in the condition of rest, as shown inFig. 6, a shunt path across resistance 21 is closed by the armature ofrelay 26. Under this condition, the power amplifier 31 will function totransmit marking energy. However, a spacing signal eflects the operationof the armature of relay 26 to its right-hand contact thereby openingthe shunt across resistance 27. This allows resistance 2'! to change thecurrent relationships in the power amplifier 3'! to make its grid somuch more negative with respect to its cathode that the plate currentwill be reduced to zero and no si naling energy will be supplied to thetransmitting antenna 39. This condition exists until a marking signaleffects the operation of the armature of relay 25 to its left-handcontact thereby again closing the shunt path across resistance 2! toallow the power amplifier 31 to transmit marking energy. Thus, the relay26 selectively suppresses one saw-tooth frequency modulation cycle ofthe.

wave for each spacing signal. This enables the radio transmittingstation of Fig. 6 to transmit radio-telegraph signals in the form of acontinuous wave of radiant energy which has its frequency modulated overa wide band intermittently in a saw-tooth manner in accordance with thesignals to be transmitted.

The characteristics of the radio telegraph signals transmitted by thetransmitting system of Fig. 6 are illustrated in Fig. 1 in which F0represents the width of the wide band over which the frequency of thecontinuous wave is modulated. Tr represents the duration of each unit ofa permutation code signal, all such units being of equal duration. As isindicated in Fig. 1, each sawtooth modulation of the frequency of thecontinuous wave represents a marking signal and each time interval Trbetween the teeth of the sawtooth frequency modulations represents aspacing signal.

The radio receiving vand recording station shown in Fig. 7 is providedwith an antenna 5| for receiving the signals, illustrated in Fig. 1,that are transmitted by the transmitting system of Fig. 6. The signalsreceived by the antenna 51 are delivered to a radio receiver 63 which inturn supplies them to a detector 52 where they are combined withoscillations from an electronic beating oscillator 53. The frequency ofthe oscillations generated by the oscillator 53 is varied over a wideband in a saw-tooth manner similar to the saw-tooth frequency wavegenerated in the transmitting system, of Fig. 6. This is accomplished bymeans of the variable condenser 55 which is electrically connected intothe plate circuit of the oscillator 53. The capacitance of the capacitor54 is varied by means of a sliding member 55 which has one end attachedto the movable plates of condenser 54 and which carries at its other enda cam follower 56. Cam follower 56 is adapted to ride along the edge ofa rotating cam 51 and is held in physical contact therewith by means ofa coiled spring 58. The periphery of the cam 51 is designed to vary thecapacitance of condenser 54 in such a way as to cause the frequency ofthe oscillations generated by oscillator 53 to vary in a saw-toothmanner.

Cam 5'! is fixedly mounted on the drive shaft 59 of a rotary distributor60. The drive shaft 59 is driven by means of a gear box 62 which ismechanically coupled to the cam barrel 82 in the teletypewriter 16. Thisteletypewriter 16 is of the type described in Patent 1,904,164 grantedApril 18, 1933, to S. Morton et al., and it is also provided with aselecting magnet 15. The disclosure of this Morton et al. patent isincorporated herein by reference as a part of this specification. Therotation of the cam barrel 82 is synchronized with the rotation of thebrush arm iii of the trans- Since the gear box 62 is designed to rotatethe drive shaft 59 and the brush arm 78 seven times as fast as the driveshaft 12 and the brush arm 18 at the transmitting station of Fig. 6, thesaw-tooth frequency variations of oscillator 53 will consequently besynchronized with those of oscillator 28 at the transmitting station ofFig. 6. The resulting synchronized oscillations of oscillator 53,illustrated by the saw-tooth wave represented by the heavy line in Fig.2, are supplied by a coil 66 to the detector 52.

The signaling radiant energy transmitted by the antenna 39 of Fig. 6travels through the ionosphere over a multiplicity of space paths havingslightly different travel times which produce different delays in thearrival of the duplicate signal waves at the receiving antenna of Fig.7.

Consequently, the radio receiver 63 will supply the detector 52 with amultiplicity of duplicate signal waves having slight time separations.Two of these waves are represented by the light lines in Fig. 2. Thesewaves are combined in the detector 52 with oscillations from theoscillator 53. As is indicated in Fig. 2, the received duplicate signalwaves, due to their time diversity characteristic, will, in general, beslightly delayed with respect to the sweep frequency generated by thebeating oscillator 53.

Since the degree of delay is not the same for all the received duplicatesignal waves, the output of the detector 52 will consist of a pluralityof concurrent waves of different audio frequencies. For example, Fig. 3shows a wave having a frequency f which represents the output of thedetector 52 due to one of the received duplicate signal waves shown inFig. 2, and Fig. 4 shows a a wave having a frequency 1 simultaneouslyproduced by the detector 52 in response to the other received duplicatesignal wave shown in Fig. 2. The frequencies of the waves of Figs. 3 and4 difier by amounts proportional to the differences in the travel timesof the space paths traversed by their corresponding duplicate signalwaves. Thus, the instantaneous output of the detector 52 will consist ofa plurality of concurrent waves of various audio frequencies extendingfrom if to in in accordance with the instantaneous delay characteristicsof the duplicate signal waves received at any one time by the antenna5|.

The detector 52 has its output connected to an audio amplifier 67 of anysuitable design. It is desirable to vary the gain of the amplifier 57 bymeans of an appropriate automatic volume control circuit indicated inFig. 7 by the reference designation AVC. The output of the amplifier 57is connected to a plurality of parallel filters F1, F2, F3, F4, F5 andFn, each having a narrow pass-band, for separating the concurrent audiofrequency waves into a plurality of different frequency bands. Oneadvantage resulting from the use of these narrow pass-bands is thatnoise and interfering waves are efiectively suppressed because the bandwidths are insufificient to allow the varying frequency currentsproduced by the noise, or interfering waves, and the variable frequencyoscillator to build up to full amplitude. As was stated above, eachfilter has limits different from all the other filters and thepass-bands of all the filters are continuously distributed over thefrequency range of the signaling energy applied thereto. In a preferredembodiment of the invention in which the pass-bands of the filters arecontinuously distributed in an overlapping manner over a range of 1140cycles, forty overlapping filters are used, each having a pass-band of57 cycles.

Each of the filters F1 to Fn, inclusive, has its output-separatelyconnected to a difierent one of a plurality of rectifiers R1, R2, R3,R4, R5 Rn in the manner illustrated in Fig. '7 for transforming thefilter outputs into unidirectional potentials proportional to themagnitudes of their corresponding audio frequenc waves. The resultingrectified filter outputs are of positive polarity with respect to groundand are individually supplied to a scanning device represented by thescanning-distributor 6U.

The scanning distributor 63 is shown, by way of example, as being amechanical rotary distributor. It is to be understood that the inventionis not limited to this type of distributor but can employ various othertypes of distributors,

s time.

such as the electronic distributors disclosed in Patent 2,185,693granted January 2, 1940, to P. Mertz, and'Patent 2,217,774 grantedOctober 15, 1940, to A. M. Skellett. The distributor 60 comprises asolid commutator ring 68 and a segmented commutator ring 69.

The segmentedcommutator ring 69 comprises a plurality of segments Hwhich are insulated from each other. The number of the segments 7! isthe same as the number of the filters F1 to Fn, inclusive. Each of therectifiers R1 to Rn, inclusive, has its output separately connected to adifferent one of the segments H in the manner illustrated in Fig. 7. Thedistributor 60 also includes a rotary brush arm 70 which is equippedwith two brushes adapted to sweep over the rings 68 and 59. The brusharm 10 is fixedly mounted on the drive shaft 59 for rotation therewith,the

1 period of rotation being Tr.

In accordance with the principles of start-stop telegraph operation, thebrush arm 10'does not rotate during idle periods since neither the driveshaft 59 nor the cam barrel 82 are rotating at this Also, during idleperiods, the cam follower 56 remains practically at the top'of the cam51 so that the oscillator 53 will be generating constant frequency waveshaving approximately the maximum'frequency. In order to begin therotation of the brush arm 10, a start signal is transmitted which, aswas described above, is a wave of steady frequency. At the receivingstation, the steady wave constituting the start si nal is combined inthe detector 52 with the steady waves from oscillator 53. The resultingoutput wave of the detector 52 will have a constant frequency and isapplied, after amplification, to the start filter Fs which isa narrowband-pass filter designed to pass waves having'this particularfrequency, f5. The output of filter Fs is rectified by a rectifier Rsand the resulting output energy is delivered to the grid of a triode 6|to render tube 6! conductive. The current which now flows in the cathodereturn circuit of tube 6| passes through the winding of an electricrelay 65 thereby energizing relay 65 and causing it to operate itsarmature. This opens the circuit extending from battery 85 to thecontact 14 leading to the selector magnet 75. The removal of currentfrom selector magnet l5 causes it to start the teletypewriter H3 in themanner described in the abovementioned Morton et al. patent therebyinitiating rotation of the cam barrel 82. barrel 82 causes the driveshaft 59 to rotate, as was explained above, and this, in turn, causesthe brush arm 70 to rotate and sweep over the commutator rings 68 and 69for scanning the rectified filter outputs.

In order that all the rectified filter outputs may be scanned during thetime that their values are Rotation of cam at, or near, a maximum, thesegments 1| are disposed along only four-tenths of the ring 69 as isindicated in Fig. 7, the remaining portion of the ring 69 beingdepressed so as not to contact its brush. Thus, all the rectified filteroutputs are scanned during a period of time equal to four- .tenths ofthe period Tr. This is indicated in Fig. 5 as is explained hereinafter.In a preferred embodiment of the invention, each rectified filter outputis scanned, or sampled, for a period of 0.3 millisecond.

To determine whether any of the scanned rectified filter outputs have amagnitude in excess of a preassigned minimum value, the common ring 68is connected to a trigger device, such as the control grid of a gasdischarge tube 13. A negative biasing voltage from battery 12 is alsosupplied to the control grid of the tube 13. The value of the biasingvoltage from battery 12 is sufficiently large to bias the tube 13 beyondcut-off. Tube 73 has its cathode return circuit connectable over acontact 64 to the selecting magnet "l5 of the receiving teletypewriter'16. Contact 64 is adapted to be repeatedly opened and closed inresponse to the closing and opening of the contact 14 by a cam 11fixedly mounted on the drive shaft 59 for rotation therewith. Theperiphery of the cam T! is so designed as to hold the contact 14 openduring four-tenths of the period of rotation of the cam Tl, this beingthe same period in which the brush arm 10 sweeps over the segments ll.During this period the contact 64 will be closed so that the circuitextending from the cathode of tube 13 to the selecting magnet 15 will beclosed while the brush arm 70 is sweeping over the segments H and willbe opened during the remainder of the period of rotation of the brusharm 10.

Whenever the brush arm 70 sweeps over one of the segments H which issupplied with a rectified filter output having a value sufficientlylarge to overcome the biasing voltage from battery 12, the tube l3 willbreak down and become conductive. This will cause current to fiow overthe cathode return circuit of tube 13 and over the contact 64 toenergize the selecting magnet 15 in the teletypewriter 16 therebyeffecting the recording of a marking signal by the teletypewriter 16.After the tube 13 becomes conductive, it remains conductive until therotation of cam 11 allows the contact 74 to close thereby openingcontact E4 and the circuit from the cathode of tube 73 to the selectormagnet 75. Consequently, if several of the segments H are supplied withstrong currents, tube 13 will break down when the brush arm l sweepsover the first of these segments and the currents supplied to the othersegments will perform no useful function. If, during one sweep cycle ofthe brush arm 10, none of the rectified filter outputs are sufiicientlylarge to overcome the biasing voltage from the battery 12, then the gasdischarge tube 13 will remain non-conductive andno current will be sentto the selecting magnet 15 which will therefore cause the teletypewriter16 to record a spacing signal. Thus, the rectified filter outputs arescanned in respect to their magnitudes to determine whether any of theirmagnitudes are in excess of a preassigned value; namely, the value ofthe biasing voltage from the battery 12.

This scanning feature isillustrated in Fig. in which the horizontal,line B-Brepresents the value of the steady biasing voltage from thebattery 13. The rectified output of one of the filters is represented bythe curve SW. One complete scanning cycle of all the rectified filteroutputs is performed during each of the time intervals indicated by theshort lines tl-tZ, t3t4, i5ie, and t7-t8. As was stated above, the valueof all the rectified filter outputs will be at, or near, a maximum ifmarking signals have been received during these particular timeintervals. It can be seen in Fig. 5 that the value of the particularrectified filter output'represented by the curve SW fails to exceed thevalue of the bias voltage B-B during the scanning period indicated bythe line Let t=minimum delay difference between the paths which are tobe resolved.

T=spread in delay differences likely to be encountered.

A=band width of the band-pass filters F1,

F2 Fn. B=total audio band covered by the band-pass filters F1, F2 Fn.

Fo=width of the band over which the frequency of the continuous wave ismodulated.

Tr=duration of each unit of a permutation code signal.

In order to separate waves of t delay difference by means of theband-pass filters F1, F2 F11, it is necessary to have, approximately:

EF0=2A (1) In order to permit the filter output to build up to a flatoutput during the time Tr, it is necessary to have approximately:

The total audio band to be provided is: T B=TfF =f1l7O f where f1 and fnare the values of the peak frequen'cies, or the mid-band frequencies, ofthe Waves passed by the filters F1 and Fn.

Typical values might be:

t= 0.3 millisecond T=3.0 milliseconds Tr=30 milliseconds, for printintelegraphy.

Whence:

A=57 cps. Fo='11400 cps.

' B=1140 cps.

What is claimed is:

1. A radio transmitting and receiving system including. in combination atransmitting station adapted to'transmit awave of radiant energy havingits frequency modulated over a Wide band v From 1) and (2) the followingvalue is obtained:

intermittently in a saw-tooth manner in accordance with signals, areceiving station adapted to receive the transmitted wave in the form ofa plurality of duplicate waves having difierent delays characteristicoft-he different travel times of the various space paths traversed bysaid duplicate waves, said receiving station comprising a beatingoscillator having'its frequency modulated over a wide band in asaw-tooth manner, a detector for combining said duplicate received waveswith the output of the beating oscillator, a plurality of narrowband-pass filters, each of said, filters having limits different fromall the other filters, connecting means for connecting the output of thedetector to each of said filters in parallel, a plurality of rectifiers,circuit means for'connecting the output of each of said filters to adifferent one of said rectifiers, a scanning device, and connectingmeans for separately connecting the output of each of said rectifiers tothe scanning device, said scanning device being adapted to scan each ofsaid rectified filter outputs in respect to their magnitudes fordetermining Whether at least one of the rectified filter outputs has amagnitude in excess of a preassigned minimum value.

2. A radio communication system including in combination a transmittingstation adapted to transmit a wave of radiant energy having itsfrequency modulated over a wide band in a sawtooth manner intermittentlyin accordance with at least two types of signals, each toothrepresenting one 'type of signal and each interval between the teethrepresenting a different type of signal, an antenna for transmittingsaid intermittently modulated wave, a receiving station having areceiving antennaadapted to receive the transmitted, wave in the form ofa plurality of duplicate waves having different arrival times inaccordance with the different travel times of the various space pathstraversed by said duplicate waves, said receiving station comprising abeating oscillator having its frequency cyclically modulated over a wideband in a saw-tooth manner, a detector for combining said duplicatereceived waves with the, output of the beating oscillator, a pluralityof narrow band-pass filters each having limits different from all theothers, connecting means for connecting the output of the detector toeach of said filters in parallel, rectifying means for separatelyrectifying the output of each of said filters, a scanning device adaptedto scan each of said rectified filter outputs in respect to theirmagnitudes for determining whether at least one of the rectified filteroutputs has a magnitude in excess of a preassigned minimum value, arecording device, and selective operating means for selectively causingthe recording device to record one type of signal when all of thescanned rectified filter outputs have a magnitude less than saidpreas'signed minimum value and to record another type of signal when atleast one of the scanned rectified filter outputs has a magnitudegreater than said 'preassig'ned minimum value.

3. A radio communication system comprising in combination a radiotransmitting station having generating means for generating, a'lwave,means for modulating the frequency of said wave over a wide band in asaw-tooth'manner, radiating means for radiating said frequency modulatedwave through space, and receiving means for receiving said frequencymodulated wave over a plurality of different space paths havingdifferent travel times, said receiving means including detecting meansfor producing a plurality of concurrent audio frequency waves Whosefrequencies differ by amounts proportional to the differences in thetravel times of said diiferent space paths.

4. A radio communication system comprising in combination a radiotransmitting station having generating means for generating a wave,means for modulating the frequency of said wave over a wide band in asaw-tooth manner, radiating means for radiating said frequency modulatedwave through space, and receiving means for receiving said frequencymodulated wave over a plurality of different space paths havingdifferent travel times, said receiving means including a beatingoscillator for generating a wave, means for varying the frequency of thewave generated by said beating oscillator over a wide band in asaw-tooth manner, detecting means for combining the variable frequencywave produced by said beating oscillator with said received waves forproducing a plurality of concurrent audio frequency waves whosefrequencies differ by amounts proportional to the differences in thetravel times of said different space paths, and a plurality of filtersfor separating said concurrent audio frequency waves into a plurality ofdifferent frequency bands.

5. A radio communication system comprising in combination a radiotransmitting station having generating means for generating a wave,means for modulating the frequency of said wave over a wide band in asaw-tooth manner, radiating means for radiating said frequency modulated wave through space, receiving means for receiving said frequencymodulated wave over a, plurality of different space paths havingdifferent travel times, detecting means for producing a plurality ofconcurrent audio frequency waves whose frequencies differ by amountsproportional to the diiferences in the travel times of said differentspace paths, a plurality of filters for separating said concurrent audiofrequency waves into a plurality of different frequency bands, aplurality of rectifiers for separately rectifying the filter outputs,andscanning means for cyclically scanning the rectified filter outputs.

6. A radio communication system comprising in combination a radiotransmitting station having generating means for generating a wave,means for modulating the frequency of said wave over a wide band in asaw-tooth manner, radiating means for radiating said frequency modulatedWave through space, receiving means for receiving' said frequencymodulated wave over a plurality of different space paths havingdifferent travel times, detecting means for producing a plurality ofconcurrent audio frequency waves whose frequencies differ by amountsproportional to the differences in the travel times of said differentspace paths, a plurality of filters for separating said concurrent audiofrequency waves into a plurality of different frequency bands, aplurality of rectifiers for separately rectifying the filter outputs,scanning means for cyclically scanning the rectified filter outputs, arecording device, and, operating means for selectively operating therecording device in accordance with the scanning. operations performedby said scanning means.

'7. A radio communication'systein comprising in combination a radiotransmitting station having generating means for generating a wave,means for incdulatin'githe frequency of said wave over a wide band in asaw-tooth manner, radiat ing means for radiating said frequencymodulated wave through space, receiving means for receiving saidfrequency modulated wave over a plurality of difierent space pathshaving different travel times, detecting means for producing a pluralityof concurrent audio frequency waves whose frequencies differ by amountsproportional to the differences in the travel times of said differentspace paths, a, plurality of filters for separating said concurrentaudio frequency waves into a plurality 01 different frequency bands, aplurality of rectifiers for transforming the filter outputs intounidirectional potentials proportionalto the magnitudes of theircorresponding audio frequency waves, scanning means for sampling saidunidirectional potentials, a trigger device, and operating means foroperating the trigger device in accordance with the sampling operationsperformed by said scanning means.

8. A radio communication system comprising in combination a radiotransmitting station having generating means for generating a wave,means for modulating the frequency of said wave over a wide band in asaw-tooth manner, radiatin means for radiating said frequency modulatedwave through space, receiving means for receiving said frequencymodulated wave over a plurality of difierent space paths havingdifferent travel times, detecting means for producing a plurality ofconcurrent audio frequency waves whose frequencies differ by amountsproportional to the differences in the travel times of said differentspace paths, a plurality of filters for separating said concurrent audiofrequency waves into a plurality of different frequency bands, aplurality of rectifiers for transforming the filter outputs intounidirectional potentials proportional to the magnitudes of theircorresponding audio frequency waves, scanning means for cyclicallyscanning said unidirectional potentials in respect to their magnitudesfor determining whether at least one of said scanned potentials has amagnitude in excess of a preassigned minimum value, a normallynon-conductive trigger tube, operating means for rendering said triggertube conductive whenever the magnitude of at least one of said scannedpotentials is greater than said preassigned minimum value, and controlmeans for rendering said trigger tube nonconductive after the completionof each of said scanning cycles.

9. A radio communication system for communicating at least two types ofsignals between a transmitting station and a receiving station, saidsystem comprising in combination a transmitting station havinggenerating means for generating a wave, means fo cyclically modulatingthe frequency of said wave over a wide band in a saw-tooth manner,control means for selectively suppressing one saw-tooth frequencymodulation cycle of said wave for each signal of one type, radiatingmeans for radiating said wave through space, receiving means forreceiving said wave over a plurality of different space paths havingdifferent delay characteristics, detecting means for producing aplurality of concurrent audio frequency waves whose frequencies differby amounts proportional to the differences between said different delaycharacteristics, a plurality of filters for separating said concurrentwaves into a plurality of different frequency bands, rectifying meansfor separately converting said difierent frequency hands intounidirectional potentials, scanning means for scanning saidunidirectional potentials in respect to their magnitudes for determiningwhether at least one of said scanned potentials has a magnitude greaterthan a proassigned minimum value, a recording device, and selectiveoperating means for selectively causing the recording device to record asignal of one type when all of said scanned potentials have a magnitudeless than said preassigned minimum value and to record a signal ofanother type when at least one of said scanned potentials has amagnitude greater than said preassigned minimum value.

10. The method of radio communication which comprises generating acontinuous wave of electrical energy, modulating the frequency of saidwave over a wide band in a saw tooth manner, intermittently suppressingselected saw-tooth frequency modulation cycles of said wave, radiatingthe resulting intermittent frequency modulated wave through space,receiving said wave over a diversity of space paths having differentdelay characteristics, and producing a plurality of concurrent audiofrequency waves having frequencies differing by amounts proportional tothe differences in said different delay characteristics.

11. The method of radio communication which comprises generating acontinuous wave of electrical energy, modulating the frequency of saidwave over a wide band in a saw-tooth manner, intermittently suppressingselected saw-tooth frequency modulation cycles of said wave, radiatingthe resulting intermittent frequency modulated wave through space,receiving said wave over a diversity of space paths having differentdelay characteristics, producing a plurality of concurrent audiofrequency waves having frequencies differing by amounts proportional tothe differences in said different delay characteristics, converting saidaudio frequency waves into unidirectional potentials, cyclicallyscanning said unidirectional potentials, and selectively operating arecording device in accordance with said cyclical scanning operations.

CARL B. H. FELDMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Peterson Anr. 9 1929

